Construction of a 70cm band
Mini Kits EME221-70CM pre-a
mp kit - November 2020
The
Mini Kits EME221-70CM kit is a high performance masthead mount
receiver pre-amplifier that is specifically designed for the 70cm band
(430
to 450MHz). See below specification.
I embarked on this project as I had had some good success working
through the ISS repeater on a number of occasions, however felt that
the receiver sensitivity of the Yaesu FT-736R 70cm module was not as
good as would be ideal and a masthead mounted
pre-amplifier would be a ideal solution. A number of fully constructed commercial
pre-amps were considered, but finally the
Mini Kits EME221-70CM
pre-a
mp kit was chosen based on the advertised performance specifications and
dramatically more competitive price.
The idea of
the constructing a quality kit was also more appealing, if not a
little daunting as I had limited experience with SMD (Surface Mount
Device) construction; therefore best to get some experience.
The kit notes do say 'Do not construct this Kit unless you are very
experienced with surface mount construction'
Specifications
Frequency
Range:
430
to 450MHz ( BW 20MHz @ 1dB )
Band
Rejection:
Typically
>35dB @ 145MHz, >60dB @ 900MHz
Gain
Adjustable:
Typically
+29dB Max, 20dB Min @ 435MHz
Noise
Figure:
1dB
( Typically <0.9dB @ 435MHz )
Through
Loss:
0.3dB
@ 435MHz ( Bypass Mode )
RF
Power Handling:
50W
CW ( Un-Sequenced ) 100W PEP ( Sequenced )
Minimum
RF Input:
+20dBm
( 100mW ) To activate the RF sensing circuit
Operating
Voltage:
+11
to 15vdc ( +12vdc @ 220mA Recommended )
Board
Size:
80mm
L x 84mm W x 17mm H ( with TNC connectors )
Photo 1 Completed
assembly of the PCB
The construction notes
were excellent and the construction generally went smoothly except for
dropping a small SMD resistor on the floor; it is still there, but
will never been seen again. Once the replacement resistor was sourced
the board construction was completed.
The
pre-amp includes RF sensing circuity and relays that can operated up to
50W CW / FM and 100W pep SSB, however in this installation the DC
power and switching will be injected into the coax cable via a Bias
Tee and switched by the Yaesu
FT-736R STBY Jack.
The tune up and
performance testing was completed with a NanoVAN in LOGMAG mode that revealed
the amplifier was producing 28.77dB gain at the target frequency of 436MHz.
The NanoVNA required a 20dB attenuator between the CH0 port and the
input to the amplifier to avoid overloading the amplifier.
Photo 2 Amplifier
gain measurements with a NanoVNA.
The NanoVNA also showed
that the 20MHz bandwidth (All the 70cm amateur band) was with 1dB of
the peaked frequency of 437MHz. The gain adjustment range was also
measured at +28.77dB maximum to approximately +19dB minimum, consistent
with the specifications. The alignment of the amplifier is by tuning
of the C2 trimmer for maximum gain at the desired frequency.
While
on the bench a number of satellite beacons were monitored with and
without the amplifier switched in. In one instant the satellite beacon
was barely discernable in the noise and was bought up to S6 with the
amplifier, not a totally scientific measurement as satellite signals
are highly variable, however this did give a dramatic demonstration of what
is to be expected.
Fantastic
kit, performs as specified, designed in Australia, look fantastic and
enjoyable to construct.
Video of the construction
and testing of a masthead pre-amplifier kit designed for the 70cm band
(430 to 450MHz). The video includes an overview of the kit, examples
of my first attempts at the installation of surface mound devices and
final performance testing.
Video is of a NanoVNA being used to measure the gain of a 70cm band pre-amplifier including the general set up and VNA calibration. This video is an exert from an earlier video detailing the full construction video of a Mini-Kits 70cm band
pre-amplifier kit.
Video
of on air testing using the amateur satellite RS-44 beacon.
